His mother managed to negotiate permission to cross the river, all he had to do was marry a girl he didnt know (nothing new for highborn girls and boys). Yet, Robb decided to follow his heart, betrayed his allies, and went back on a promise. When the time comes for Robb to marry into the Freys he is already married (Jeyne or Talisa) so he offers his uncle Edmure instead. However, the Freys are not satisfied. Lord Walder Frey's wife has just passed so he demands Catelyn becomes his (eighth?) wife. Robb is appalled, but Catelyn (who understands politics much better than her son) sees there is no way around this if they want to keep the alliance. Robb finally sees how costly his little adventure has been...

Alt: if not Walder Frey, some other disgusting Frey will do too

http://asoiafkinkmeme.livejournal.com/22515.html?thread=16541939#t16541939